DVDStyler 3.0.4 comboboxes of "properties of VMGM menu" drawn transparent erroneously unless moving offscreen and back

This ticket is a continuation of CORE-14701.

Reproduction steps

• Install DVDStyler 3.0.4 from https://sourceforge.net/projects/dvdstyler/files/dvdstyler/3.0.4/DVDStyler-3.0.4-win32.exe/download?use_mirror=kent
• Run the main app
• right click the properties of "VMGM menu" in the bottom left
• the properties dlg does open up

Observed result
The comboboxes are not drawn initially and therefore the window in the background "shines through". affected.png
0.4.15-dev-1605-gab45955_affected.log
0.4.15-dev-1605-gab45955_affected.webm

Expected result
The comboboxes should be drawn initially, like in ok.png
I created ok.png by dragging the Dlg offscreen-and-back.